One use of an analog voltage-controlled phase shifter is to align the phase of a narrow-band signal such as a recovered clock signal with the incoming data stream such that decisions are made at the proper times. Another use is to phase modulate a carrier signal. There are many other uses and applications for analog phase shifters as they can be used at many different frequencies.
In one application, the phase shifter currently in use in lightwave regenerators requires a branch-line quadrature hybrid in combination with gallium arsenide hyperabrupt varactor diodes to obtain the required range of phase shift. These diodes have the disadvantages that they are expensive, they are subject to large variations in center capacitance and capacitance slope Vs. voltage, and proper operation of the phase shifter relies on well-matched diodes.